CI: add changelog check (#2766)

* Added changelog item check

Fetch the base branch and diff correctly

- See: https://github.com/actions/checkout/issues/160

Improve organisation

Add some echo for easier debugging

Fix bad variable syntax

Clarify the double-print in the success case

* Write to changelog

* Don't block dependabot PRs

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 - Upgrade to Rust 2021 edition #2748 (@cyqsimon)
 - Refactor and cleanup build script #2756 (@cyqsimon)
+- Checks changelog has been written to for PRs in CI #2766 (@cyqsimon)
 
 ## Syntaxes